Load--Line Analysis
Load
Electronic Circuits
The load line plots all possible
combinations of diode current (ID)
and voltage (VD) for a given circuit.
The maximum ID equals E/R, and
the maximum VD equals E.
Prof. Nizamettin AYDIN
naydin@yildiz.edu.tr
http://www.yildiz.edu.tr/~naydin
The point where the load line and
the characteristic curve intersect is
the Q-point, which identifies ID and
VD for a particular diode in a given
circuit.
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Series Diode Configurations
Series Diode Configurations
Forward Bias
Constants
• Silicon Diode: VD = 0.7 V
• Germanium Diode: VD = 0.3 V
Reverse Bias
Diodes ideally behave as open circuits
Analysis
• VD = E
• VR = 0 V
• ID = 0 A
Analysis (for silicon)
• VD = 0.7 V (or VD = E if E < 0.7 V)
• VR = E – VD
• ID = IR = IT = VR / R
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Parallel Configurations
Half--Wave Rectification
Half
V = 0.7 V
D
The diode only
conducts when it is
forward biased,
therefore only half
of the AC cycle
passes through the
diode to the
output.
V
=V
= V = 0.7 V
D1
D2
O
V = 9.3 V
R
I
I
R
=
D1
E−V
D = 10 V − .7 V = 28 mA
R
.33kΩ
=I
D2
=
28 mA
2
4
= 14 mA
The DC output voltage is 0.318Vm, where Vm = the peak AC voltage.
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PIV (PRV)
Full
Full--Wave Rectification
Because the diode is only forward biased for one-half of the AC cycle, it is
also reverse biased for one-half cycle.
The rectification process can be improved by
using a full-wave rectifier circuit.
It is important that the reverse breakdown voltage rating of the diode be
high enough to withstand the peak, reverse-biasing AC voltage.
Full-wave rectification produces a greater
DC output:
PIV (or PRV) > Vm
•
•
•
PIV = Peak inverse voltage
PRV = Peak reverse voltage
Vm = Peak AC voltage
•
•
Half-wave: Vdc = 0.318V
0.318Vm
Full-wave: Vdc = 0.636
0.636V
Vm
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Full
Full--Wave Rectification
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Full
Full--Wave Rectification
CenterCenter-Tapped Transformer Rectifier
Bridge Rectifier
•
•
Requires
• Two diodes
• Center-tapped transformer
Four diodes are connected in a
bridge configuration
VDC = 0.636Vm
VDC = 0.636Vm
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Diode Clippers
Summary of Rectifier Circuits
Rectifier
Ideal VDC
Realistic VDC
Half Wave Rectifier
VDC = 0.318Vm
VDC = 0.318Vm – 0.7
Bridge Rectifier
VDC = 0.636Vm
VDC = 0.636Vm – 2(0.7 V)
Center-Tapped Transformer
Rectifier
VDC = 0.636Vm
VDC = 0.636Vm – 0.7 V
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The diode in a series clipper “clips”
any voltage that does not forward
bias it:
• •A reverse-biasing polarity
•A forward-biasing polarity less than
0.7 V (for a silicon diode)
Vm = peak of the AC voltage.
In the center tapped transformer rectifier circuit, the peak AC voltage
is the transformer secondary voltage to the tap.
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2
Biased Clippers
Parallel Clippers
Adding a DC source in
series with the clipping
diode changes the
effective forward bias of
the diode.
The diode in a parallel clipper
circuit “clips” any voltage that
forward bias it.
DC biasing can be added in
series with the diode to change
the clipping level.
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Summary of Clipper Circuits
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Summary of Clipper Circuits
more…
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Clampers
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Biased Clamper Circuits
The input signal can be any type
of waveform such as sine, square,
and triangle waves.
A diode and capacitor can be
combined to “clamp” an AC
signal to a specific DC level.
The DC source lets you adjust
the DC clamping level.
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3
Zener Diodes
Summary of Clamper Circuits
The Zener is a diode operated
in reverse bias at the Zener
Voltage (Vz).
• When Vi ≥ VZ
– The Zener is on
– Voltage across the Zener is VZ
– Zener current: IZ = IR – IRL
– The Zener Power: PZ = VZIZ
• When Vi < VZ
– The Zener is off
– The Zener acts as an open circuit
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Zener Resistor Values
Voltage--Multiplier Circuits
Voltage
If R is too large, the Zener diode cannot conduct because the available amount of
current is less than the minimum current rating, IZK. The minimum current is
given by:
I Lmin = IR − I ZK
Voltage multiplier circuits use a combination of diodes and
capacitors to step up the output voltage of rectifier circuits.
•
•
•
The maximum value of resistance is:
R Lmax =
VZ
I Lmin
Voltage Doubler
Voltage Tripler
Voltage Quadrupler
If R is too small, the Zener current exceeds the maximum current
rating, IZM . The maximum current for the circuit is given by:
VL
V
= Z
RL
RL min
The minimum value of resistance is:
I L max =
RL min =
RVZ
Vi − VZ
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Voltage Doubler
Voltage Doubler
This half-wave voltage doubler’s output can be calculated by:
•
Positive Half-Cycle
o D1 conducts
o D2 is switched off
o Capacitor C1 charges to Vm
•
Negative Half-Cycle
o D1 is switched off
o D2 conducts
o Capacitor C2 charges to Vm
Vout = VC2 = 2Vm
Vout = VC2 = 2Vm
where Vm = peak secondary voltage of the transformer
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Practical Applications
Voltage Tripler and Quadrupler
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•
Rectifier Circuits
– Conversions of AC to DC for DC operated circuits
– Battery Charging Circuits
•
Simple Diode Circuits
– Protective Circuits against
– Overcurrent
– Polarity Reversal
– Currents caused by an inductive kick in a relay circuit
•
Zener Circuits
– Overvoltage Protection
– Setting Reference Voltages
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